Rock bits, referred to more generally as drill bits, are used in earth drilling. Two predominant types of rock bits are roller cone rock bits and shear cutter bits. Shear cutter bits are configured with a multitude of cutting elements directly fixed to the bottom, also called the face, of the drill bit. The shear bit has no moving parts, and its cutters scrape or shear rock formation through the rotation of the drill bit by an attached drill string. Shear cutter bits have the advantage that the cutter is continuously in contact with the formation and see a relatively uniform loading when cutting the gage formation. Furthermore, the shear cutter is generally loaded in only one direction. This significantly simplifies the design of the shear cutter and improves its robustness. However, although shear bits have been found to drill effectively in softer formations, as the hardness of the formation increases it has been found that the cutting elements on the shear cutter bits tend to wear and fail, affecting the rate of penetration (ROP) for the shear cutter bit.
In contrast, roller cone rock bits are better suited to drill through harder formations. Roller cone rock bits are typically configured with three rotatable cones that are individually mounted to separate legs. The three legs are welded together to form the rock bit body. Each rotatable cone has multiple cutting elements such as hardened inserts or milled inserts (also called xe2x80x9cteethxe2x80x9d) on its periphery that penetrate and crush the formation from the hole bottom and side walls as the entire drill bit is rotated by an attached drill string, and as each rotatable cone rotates around an attached journal. Thus, because a roller cone rock bit combines rotational forces from the cones rotating on their journals, in addition to the drill bit rotating from an attached drill string, the drilling action downhole is from a crushing force, rather than a shearing force. As a result, the roller cone rock bit generally has a longer life and a higher rate of penetration through hard formations.
Nonetheless, the drilling of the borehole causes considerable wear on the inserts of the roller cone rock bit, which affects the drilling life and peak effectiveness of the roller cone rock bit. This wear is particularly severe at the corner of the bottom hole, on what is called the xe2x80x9cgage rowxe2x80x9d of cutting elements. The gage row cutting elements must both cut the bottom of the wellbore and cut the sidewall of the borehole. FIG. 1 illustrates a cut-away view of a conventional arrangement for the inserts of a roller cone rock bit. A cone 110 rotates around a journal 120 attached to a rock bit leg 108. The cone 110 includes inserts 112 that cut the borehole bottom 150 and sidewall 155.
The inserts 115 cutting the rock formation are the focus for the damaging forces that exist when the drill bit is reaming the borehole. The gage row insert 115 at the corner of the bottom 150 and sidewall 155 is particularly prone to wear and breakage, since it has to cut the most formation and because it is loaded both on the side when it cuts the bore side wall and vertically when it cuts the bore bottom. The gage row inserts have the further problem that they are constantly entering and leaving the formation that can cause high impact side loadings and further reduce insert life. This is especially true for directional drilling applications where the drill bit is often disposed from absolute vertical.
The wear of the inserts on the drill bit cones results not only in a reduced ROP, but the wear of the corner inserts results in a borehole that is xe2x80x9cunder gagexe2x80x9d (i.e. less than the full diameter of the drill bit). Once a bit is under gage, it is must be removed from the hole and replaced. Further, because it is not always apparent when a bit has gone under gage, an undergage drill bit may be left in the borehole too long. The replacement bit must then drill through the under gage section of hole. Since a drill bit is not designed to ream an undergage borehole, damage may occur to the replacement bit, especially at the areas most likely to be short-lived and troublesome to begin with. This decreases its useful life in the next section. Because this can result in substantial expense from lost drill rig time as well as the cost of the drill bit itself, the wear of the inserts at the corner of the rolling cone rock bit is highly undesirable.
Another cause of wear to the inserts on a rock bit is the inefficient removal of drill cuttings from the bottom of the well bore. Both roller cone rock bits and shear bits generate rock fragments known as drill cuttings. These rock fragments are carried uphole to the surface by a moving column of drilling fluid that travels to the interior of the drill bit through the center of an attached drill string, and is ejected from the face of the drill bit. The drilling fluid then carries the drill cuttings uphole through an annulus formed by the outside of the drill string and the borehole wall. In certain types of formations the rock fragments may be particularly numerous, large, or damaging, and accelerated wear and loss or breakage of the cutting inserts often occurs. This wear and failure of the cutting elements on the rock bit results in a loss of bit performance by reduced penetration rates and eventually requires the bit to be pulled from the hole.
Inefficient removal of drilling fluid and drill cuttings from the bottom hole exacerbates the wear and failure of the cutting elements on the roller cones because the inserts impact and regrind cuttings that have not moved up the bore toward the surface. Erosion of the cone shell (to which the inserts or teeth attach) can also occur in a roller cone rock bit from drill cuttings when the bit hydraulics are inappropriately directed, leading to cracks and damage to the shell. Ineffective removal of drilling fluid and drill cuttings can further result in premature failure of the seals in a rock bit from a buildup of drill cuttings and mud slurry in the area of the seal. Wear also occurs to the body of the drill bit from the constant scraping and friction of the drill bit body against the borehole wall.
It would be desirable to design a drill bit that combines the advantages of a shear cutter rock bit with those of a roller cone rock bit. It would additionally be desirable to design a longer lasting drill bit that minimizes the effect of drill cuttings on the drill bit. This drill bit should also minimize the downhole wear occurring from the scraping of the drill bit against the borehole wall.
In one embodiment, the invention is a rolling cone rock bit including a body, a leg formed from the body with an attached rolling cone, and a plurality of cutting elements mounted to the backface of the leg, the plurality of cutting elements having at least one cutting element extending to the gage diameter of the drill bit. Preferably, at least a majority of the cutting tips of the cutting elements extend to gage diameter. The cutting elements may be disposed in a curved row on the leading edge of the leg. This arrangement may similarly be constructed on a second leg of the drill bit, in which case it is preferred that the cutting elements on the first leg are staggered with respect to the cutting elements on the second leg to result in overlapping cutting elements in rotated profile. The drill bit may also include a mud ramp surface for the flow of drilling fluid from the bottom of a wellbore. The cutting elements of the rolling cone cutters may be of any suitable cutting design, and may or may not extend to gage diameter. In addition, the drill bit may have inserts around its periphery to protect the body of the drill bit and to stabilize the drill bit.
In another embodiment, the invention is a rolling cone rock bit with a bit body and attached rolling cone, and a junk slot, defined by the bit body and a junk slot boundary line, wherein the junk slot has a cross-sectional area at each height along the junk slot with the area at the top of the junk slot being greater than the area at its bottom. The cross-sectional area at the top may be at least 15% greater at its top than at its bottom, it may be at least 100% greater, or it may be somewhere in the range of 15% to 600% greater. The drill bit may include a leg with a mud ramp, and the mud ramp then forms one boundary of the junk slot. The drill bit may also include a nozzle boss that forms a boundary for the junk slot, where the cross-sectional area of the junk slot is greater at the top of the mud ramp than at the bottom of the nozzle boss. The junk slot boundary may be formed by the rotational movement of an outermost point on the leg. The mud ramp may be comprised of two or more straight sections at angles from the longitudinal axis of the drill bit, or may be a set of curves such as convex or concave.
In yet another embodiment, the invention is a drill bit with at least one leg forming a mud ramp. The mud ramp has a first portion corresponding to a first angle and a second portion corresponding to a second angle, with the first angle and the second angle being different. The first portion may be a straight section, the second portion may be a straight section, the first portion may be a curve with the angle being measured with respect to a tangent to the curve at the point, and the second portion may be a curve with the angle being measured with respect to a tangent to that point.